[ticket/11543] Add unit tests for obtain_guest_count()

PHPBB3-11543
[ticket/11543] Add unit tests for obtain_users_online()

PHPBB3-11543
[ticket/11543] Add unit tests for obtain_users_online() with empty forum

PHPBB3-11543
[ticket/11543] Add unit tests for obtain_users_online_string()

PHPBB3-11543
[ticket/11543] Use correct IP addresses and inject time for correct values

PHPBB3-11543